CI: Cancel outdated PR builds when new commits are pushed to a PR branch

[DilumAluthge]

This allows us to save some CI time.

Basically, if a new commit is pushed to the branch of an existing PR, then CI will start running on that newest commit, and any CI jobs that are still running on old commits (of that same PR) will be canceled.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.48%. Comparing base (dbdef2b) to head (5f3590e).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #47   +/-   ##
=======================================
  Coverage   86.48%   86.48%           
=======================================
  Files           5        5           
  Lines         333      333           
=======================================
  Hits          288      288           
  Misses         45       45           

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